The present invention generally relates to the field of oil and gas subsurface earth formation evaluation techniques and more particularly, to methods for estimating a property of a subterranean formation and/or a property of a fracture in a subterranean formation.
Production of hydrocarbons from a subterranean formation may be affected by a number of factors including pressure, porosity, permeability, permeability functions such as relative permeabilities to water, oil, and gas, reservoir thickness and extent, water saturation, capillary pressure and capillary pressure functions. Generally, to increase production from a well bore and/or to facilitate the flow of hydrocarbons from a subterranean formation, stimulation treatment operations, such as hydraulic fracturing, may be performed.
Hydraulic fracturing typically involves introducing a high pressure fluid into the formation to create and/or enhance fractures that penetrate into the subterranean formation. These fractures can create flow channels to improve the productivity of the well. Generally, once a hydraulic fracture is induced and propagated, the release of pressure will result in the fracture closing and not maintaining a highly conductive passageway for hydrocarbon production. Thus, propping agents or “proppants” of various kinds, chemical or physical, may be used to hold the fractures open and to prevent the closing of the fractures after the fracturing pressure is released. The degree of production enhancement after such a stimulation treatment is dependant on, inter alia, the effective structure of the fracture including the height, width, and length of the fracture, as well as the conductivity of the proppant within the fracture. However, the actual structure of a fracture that is created or enhanced after such a stimulation treatment is generally not known and can only be estimated.
Various methods have been developed to attempt to estimate subterranean formation properties and/or the structure of a fracture resulting from a stimulation operation. One example of a method for estimating such properties is a pressure build-up analysis. Pressure build-up analysis refers to the analysis of data obtained from measurements of the bottomhole pressure in a well that has been shut-in after a flow period. While production of the well is stopped (e.g., for several days), the pressure build-up over time at the well is recorded. A profile of pressure against time may be created and used with mathematical reservoir models to assess the extent and characteristics of the subterranean formation and the near-wellbore area. However, to obtain such data, production from the well must generally be stopped for a significant length of time, which may be undesirable due to the associated expenses of stopping production from a well.
Another example of a method for estimating subterranean formation properties and/or the structure of a fracture resulting from a stimulation operation uses production history matching. Production history matching refers to the process of adjusting unknown parameters of a reservoir model until the model resembles the past production of the reservoir as closely as possible. Similarly, another method utilizes matching treating pressures during the fracturing treatment. When utilizing these matching methods, the accuracy of the matching depends, inter alia, on the quality of the reservoir model and the quality and quantity of pressure and production data. Once a model has been matched, it may be used to simulate future reservoir behavior.
A disadvantage associated with these methods, however, is that several different possible structures of a fracture or characteristics of a subterranean formation may yield the same result. That is, there are many possible solutions, or sets of parameter values, that can likely produce a possible match unless further constraining information is obtained.